1. Field of the Invention
The subject invention relates to electronic amplifiers, and more particularly, to a low noise gallium arsenide (GaAs) field effect transistor microwave amplifier.
2. Description of the Prior Art
As presently known, GaAs metal semiconductor field effect transistors (MESFETS) provide the lowest noise figure and are the best devices for low noise microwave amplifier applications. In such applications, a noise figure of 2.0 dB or less may be specified for particular frequencies. A drawback of present GaAs MESFET devices is that they are extremely reflective, i.e., have a very high reflection coefficient, and hence are very unstable, i.e., readily oscillate. In the design of a low noise amplifier, it is desirable to stabilize the GaAs MESFET, while retaining its intrinsic low noise characteristic.
As is well-known, amplifier stability is governed by a stability factor K. For an amplifier to be unconditionally stable its stability factor K must be kept higher than 1 in a wide frequency range. Currently, the stabilizing networks employed in the prior art do not provide both low noise operation and unconditional stability at the same time.
The prior art approaches have employed both resistive parallel feedback and resistive-capacitive parallel feedback stabilizing networks in GaAs amplifier circuits. The resistive-capacitive parallel feedback preserves the low noise characteristic of the device, but results in poor stability. Resistive-parallel feedback provides some stability, but the noise figure is an order of magnitude higher than resistive-capacitive parallel feedback.